A background art of the technical field is disclosed in JP-A-2005-307750 (PTL 1). This publication discloses an electromagnetic fuel injector that includes a coil, a fixed core, a movable core, a fuel pipe, and a spring as parts of an electromagnetic drive unit. The fuel pipe has a large-diameter circular cylindrical portion and a small-diameter circular cylindrical portion, and the small-diameter circular cylindrical portion is provided at a lower end of the large-diameter circular cylindrical portion. The fixed core is mounted on an outer circumference of the small-diameter circular cylindrical portion, and the winding coil is mounted inside of the fixed core. The fixed core has an inner circumferential core portion and an outer circumferential core portion, an inner circumferential magnetic path is formed by the inner circumferential core portion, and an outer circumferential magnetic path is formed by the outer circumferential core portion. End surfaces of the inner circumferential magnetic path and the outer circumferential magnetic path face an end surface of the movable core. Magnetic fluxes generated by the energization of the coil flow between the end surface of the inner circumferential magnetic path and the end surface of the movable core, and between the end surface of the outer circumferential magnetic path and the end surface of the movable core, and a magnetic force (suction force) corresponding to a magnetic flux density is applied therebetween (refer to paragraphs 0034 and 0036).
A lower end portion of the small-diameter circular cylindrical portion of the fuel pipe extends toward a nozzle needle from the end surfaces of the inner circumferential magnetic path and the outer circumferential magnetic path. A cover is provided such that both axial end portions of the cover are respectively laser-welded to the entire outer circumference of the lower end portion of the small-diameter circular cylindrical portion and the entire outer circumference of the large-diameter circular cylindrical portion of the fuel pipe. The fixed core and the coil are accommodated in an inner space that is bounded by the cover and the fuel pipe, and fuel is prevented from infiltrating the fixed core and the coil (refer to paragraph 0040).
In regard to positions in which the cover is laser-welded to the outer circumference of the lower end portion of the small-diameter circular cylindrical portion, and the outer circumference of the large-diameter circular cylindrical portion, while deterioration in the magnetic characteristics of the fixed core induced by heat of fusion during welding is taken into consideration, the cover is joined to portions other than a movable core-side end surface of the fixed core, specifically, portions which are relatively separated from the movable core-side end surface of the fixed core, or a separate member (refer to paragraph 0050).
PTL 1 discloses an electromagnetic fuel injector that includes a cover made of a non-magnetic material which covers only the coil between an end surface of the inner circumferential magnetic path and an end surface of the outer circumferential magnetic path. This cover prevents fuel from infiltrating the coil. The cover is made thin, and may be made of a magnetic material, and when the cover is made of a magnetic material, the thickness of a bridge portion between the end surface of the inner circumferential magnetic path and the end surface of the outer circumferential magnetic path is set to be smaller than those of other portions (refer to paragraphs 0087 to 0089 and FIGS. 11 to 13).